1. Field of the Invention
The present invention relates to an antenna device, for example, an antenna device for use in a system for non-contact communication such as NFC (near field communication), and to a communication terminal device including the antenna device.
2. Description of the Related Art
In recent years, portable terminals include a built-in antenna device for use in a non-contact communication system for a 13.56 MHz band. In order to achieve a long communication distance, this type of antenna device requires a large coil antenna, which is affixed to the inner side of a terminal case, in which a relatively large space is available. A power feed circuit (an RFIC chip) that processes an RF signal and the coil antenna are directly connected to each other via a connector or a pin.
However, the direct connection method discussed above poses a problem that the contact resistance is fluctuated in accordance with the roughness of the contact surface, oxidation, and the contact pressure, which leads to a problem in reliability such as a contact failure caused by vibration or an impact due to a fall.
Thus, Japanese Unexamined Patent Application Publication No. 2008-306689 and Japanese Patent No. 4325621 propose driving a transmission/reception antenna conducted with an RFIC chip mounted on a substrate through wiring on the substrate and a resonance antenna provided inside a terminal case, for example, with the transmission/reception antenna and the resonance antenna electromagnetically coupled to each other. With such techniques, not only can the problem described above be resolved, but also it is only necessary to couple the transmission/reception antenna to the resonance antenna, and therefore the size of the transmission/reception antenna may be reduced.
However, electromagnetic coupling between a booster coil antenna and a power feed coil antenna is not necessarily good enough because such coupling may involve generation of leakage magnetic flux or the like, which may degrade the communication characteristics.